Strategies to obtain a single enantiomer of a compound have become important in drug discovery because often one enantiomer is an effective drug while the other enantiomer has undesirable biological activity. Ideally, an asymmetric synthesis is designed to produce only the desired enantiomer. Unfortunately, more often than not, an asymmetric synthesis cannot be designed or is prohibitively expensive.
Although 1-(2,6-dichloro-3-fluorophenyl)ethanone was enantioselectively reduced to (1R)-(2,6-dichloro-3-fluorophenyl)ethanol with an enantiomeric purity of 96% enantiomeric excess (ee) using a reducing agent prepared from sodium borohydride, trimethylsilyl chloride and a catalytic amount of (S)-α,α-diphenylpyrrolidinemethanol (Jiang et al. Tetrahedron Lett., 2000, vol. 41, pp. 10281-10283), there is no available chemical synthesis for producing (1S)-(2,6-dichloro-3-fluorophenyl)ethanol, which is an intermediate in the synthesis of certain enantiomerically enriched, ether linked 2-aminopyridine analogues that potently inhibit auto-phosphorylation of human heptocyte growth factor receptor (HGFR or c-MET). Examples of c-MET (HGFR) inhibitors, their synthesis and use, can be found in U.S. patent application Ser. No. 10/786,610, entitled “Aminoheteroaryl Compounds as Protein Kinase Inhibitors”, filed Feb. 26, 2004, and corresponding international application PCT/US2004/005495 of the same title, filed Feb. 26, 2004, the disclosures of which are incorporated herein by reference in their entireties.
Our attempts to prepare (1S)-(2,6-dichloro-3-fluorophenyl)ethanol with adequate enantiomeric purity by chiral reduction of 1-(2,6-dichloro-3-fluorophenyl)ethanone using different chemical reagents, for example, (R)-2-methyl-CBS-oxazoborolidine/BH3-dimethylsulfide complex (BMS) (J. Am. Chem. Soc. 1987, vol. 109, pp. 7925); sodium borohydride, trimethylsilyl chloride and a catalytic amount of (S)-α,α-diphenylpyrrolidinemethanol (Jiang et al. Tetrahedron Lett., 2000, vol. 41, pp. 10281-10283); oxazaborolidine/BH3-dimethylsulfide complex (BMS) (J. Org. Chem. 1993, vol. 58, pp. 2880); and (−)-B-chlorodiisopinocamphenylborane (DIP-CI) (Tetrahedron Lett. 1997, vol. 38, pp. 2641), were unsuccessful.
It is known that enantiomerically pure alcohols can be produced by stereoselective reduction using biocatalysts such as enzymes or microorganisms. For example, phenylacetaldehyde reductase from Corynebacterium strain ST-10 has a broad substrate range and reduces various prochiral aromatic ketones and β-ketoesters to yield optically active secondary alcohols with an enantiomeric purity of more than 98% enantiomeric excess (ee). Itoh et al., Eur. J. Biochem., 2002, v. 269, pp. 2394-2402. According to a review on recent developments in the asymmetric reduction of ketones with biocatalysts, for example, for the reduction of ethyl 2-methyl-3-oxobutanoate, Klebseilla pneumoniae IFO 3319 out of 450 bacterial strains was found to give the corresponding (2R,3S)-hydroxy ester with >99% ee. K. Nakamura et al. Tetrahedron: Asymmetry, 2003, vol. 14, pp. 2659-2681. U.S. Pat. No. 5,310,666 discloses a Rhodotorula rubra strain, which reduces pentoxifylline to 100% to give the S-alcohol. U.S. Pat. No. 6,451,587 provides microbial asymmetric reduction processes for preparing the alcohol (R)-2-chloro-1-[6-(2,5-dimethyl-pyrrol-1-yl)-pyridin-3-yl]-ethanol from the ketone 2-chloro-1-[6-(2,5-dimethyl-pyrrol-1-yl)-pyridin-3-yl]-ethanone. U.S. Pat. Nos. 6,642,387 and 6,515,134 disclose preparation of certain optically active hydroxyethyl pyridine derivatives using microbial reduction. U.S. Pat. No. 5,391,495 discloses the stereoselective reduction of certain keto-containing sulfonamide compounds to form the corresponding hydroxyl group-containing compounds utilizing a microorganism or an enzyme capable of catalyzing the reduction. Whole cell biocatalysts were used for reducing 3,4-dichlorophenacylchloride to give the (R)- or (S)-chlorohydrine in high yields and good to high ee. Barbieri at al. Tetrahedron: Asymmetry, 1999, vol. 10, pp. 3931-3937. Nonracemic 1-phenylethylalcohol of R and S configuration and of high enantiomeric purity was obtained by bio-reduction of acetophenone in nonaqueous environment—anhydrous hexane. Zymanczyk-Duda et al., Enzyme Microbiol. Technol., 2004, vol. 34, pp. 578-582. WO 02/077258 describes preparation of certain (S)-1-(2,4-substituted-phenyl)ethanol derivatives by microbial reduction. However, the stereoselective microbial and enzymatic reduction of 1-(2,6-dichloro-3-fluorophenyl)ethanone has been unknown.
Alternatively, the mixture of enantiomers can be separated. However, mixtures of enantiomers are difficult, and often impossible, to separate because the physical properties of the enantiomers are identical towards achiral substances and can only be distinguished by their behavior towards other chiral substances. Chromatographic methods using a chiral solid phase have been utilized to separate enantiomeric mixtures, but chiral solid supports are expensive and, typically, the resolution is poor.
An alternative method of separating enantiomeric mixtures is by reacting them with a chiral reagent. In this procedure, the mixture of enantiomers react with the chiral reagent to form diastereomers which are distinguishable from each other on the basis of their properties towards achiral substances, and therefore, can be separated by techniques such as recrystallization or chromatography. This process is time consuming and results in loss of yield because it requires two additional reaction steps (i.e., one reaction to add the chiral auxiliary to the enantiomers and another reaction to remove it after the diasteriomers have been separated).
In some instances, a chiral reagent will react much faster with one enantiomer than with the other enantiomer in the enantiomeric mixture. In this case, the enantiomer which reacts faster can be removed before the other enantiomer is formed. This method also necessitates two additional reaction steps to add the chiral auxiliary and to remove it after the separation.
The methods described above cannot always be applied successfully to a particular system, and when they can be applied, they are often expensive, time consuming and result in loss of yield. Therefore, the need exists for new methods of obtaining a single enantiomer from an enantiomeric mixture.
It is known that chiral resolution of compounds can be achieved by using enzymes, such as esterases, lipases, and proteases or microrganisms. For example, U.S. Pat. No. 6,703,396 describes chiral resolution of a racemic mixture of nucleoside enantiomers based on enzymatic hydrolysis of C5′-nucleoside esters. U.S. Pat. No. 6,638,758 describes the enzymatic resolution of racemic esters of lactams using a biocatalyst, such as an enzyme or a microorganism. U.S. Pat. No. 5,928,933 discloses an enzyme with an enantiomeric excess value of 95% for N-(alkoxycarbonyl)-4-keto-D,L-proline alkyl esters as a result of extensive experiments for reaction specificity of 44 enzymes, including proteases, lipases and esterases. Preparation of certain optically active secondary alcohols by combination of enzymatic hydrolysis and chemical transformation is described, for example, in Danda et al., Tetrahedron, 1991, vol. 47, pp. 8701-8716; Vanttinen et al. Tetrahedron:Asymmetry, 1995, vol. 6, pp. 1779-1786; and Liu et al., Chirality, 2002, vol. 14, pp. 25-27.
Although biocatalysts are very useful for the separation of enantiomeric mixtures, because the selectivity for enantiomers and the optical purity of products may vary depending on the choice of an enzyme or microorganism and the chemical structures of substrates, intensive efforts are required to find combinations of biocatalysts suitable for substrates. Especially, nowhere is found a method for separating enantiomeric 1-(2,6-dichloro-3-fluorophenyl)ethanol esters using a biocatalyst.